1. Field of the Invention
The present invention relates generally to an active matrix display device, and more particularly to an active matrix display device which executes signal write by a current signal.
2. Description of the Related Art
In recent years, by virtue of such features as small thickness, light weight and low power consumption, there has been a rapidly increasing demand for flat-panel display devices which are typified by liquid crystal display devices. In particular, an active matrix display device, in which respective pixels are provided with pixel switches having functions of electrically separating turn-on pixels and turn-off pixels and holding video signals in the turn-on pixels, has widely been used in various types of displays including portable information devices, since a high image quality without cross-talk between neighboring pixels can be obtained.
As such a flat-panel active matrix display device, an organic electroluminescence (EL) display device using a self-luminous element has been attracting attention and has been vigorously researched and developed. The organic EL display device requires no backlight that hinders reduction in thickness and weight, and is suited to reproduction of a moving image because of its high-speed responsivity. Moreover, the organic EL display device has a feature that it can be used at a cold place because the luminance does not fall at low temperatures.
The organic EL display device includes, in each of pixels, an organic EL element functioning as a display element, and a pixel circuit which supplies a driving current to the display element. A display operation is performed by controlling the emission light luminance of the display element. The pixel circuit includes, for example, a driving transistor and an output switch, which are connected in series to the organic EL element, and a diode connection switch which is connected between the gate and drain of the driving transistor and retains a gate potential corresponding to a video signal. The driving transistor, output switch and diode connection switch are composed of, for example, thin-film transistors. As regards this organic EL display device, there is known a method in which image information is supplied to the pixel circuit by a current signal.
In the case of the display device in which signal supply is executed by a current signal, there is a possibility that sufficient signal supply cannot be executed due to a wiring capacitance of wiring for the signal supply. In particular, there is a problem that a display defect due to deficient write occurs when a write current value is low. In addition, multi-gradation display is performed, a write operation becomes difficult on a low-gradation side in which a set current amount is small, leading to defective display.
Jpn. Pat. Appln. KOKAI Publication No. 2004-341023 discloses an organic EL display device in which in order to prevent such write deficiency due to wiring capacitance, dual-system current signal supply is performed from a video signal driver, and a difference current is written as a video signal in a pixel. In this display device, a base current is written in a pixel circuit from a constant-current circuit via a video signal line, a gradation current is written in the pixel circuit from a source IC via the video signal line, and a difference current between the base current and the gradation current is written in the pixel circuit. The display element is driven by the difference current.
According to this structure, the value of a current that is supplied to the video signal line can freely be set, and the base current and the gradation current can be set at current values that are sufficiently higher than the wiring capacitance. As a result, by the large write current that is not affected by the wiring capacitance, a write operation can be performed with a small current that is the difference current.
In the display device with the above-described structure, however, the constant-current circuit needs to be provided for each of video signal lines, and the size of a peripheral edge portion of the display device, that is, a picture-frame portion of the display device, increases. There may be a case in which non-uniformity in display occurs in the signal line direction due to non-uniformity of a plurality of constant-current circuits. Moreover, since this device has the structure in which the difference current is derived at the time of writing the video signal, the device may easily suffer the influence of a feed-through current of the transistor at the time of light emission. In particular, since feed-through currents of both the first transistor that outputs a driving current corresponding to a base current and the second transistor that outputs a driving current corresponding to a gradation current are added, display non-uniformity, such as roughness and vertical streaks, is visually recognized and the display quality deteriorates.